The present invention relates generally to sizing guide for a bone, especially for a femur. In particular, the present invention relates to a femoral sizing guide for determining a required size of a femoral implant and for accurately positioning a cutting block on a resected distal portion of a femur in order to locate cutting planes for preparing the end of the femur to receive the femoral implant. The femoral sizing guide incorporates a rotation mechanism to allow rotational movement between two components. The present invention controls the effect of the rotation mechanism upon the determination of required femoral implant size. The present invention also relates to methods of using the femoral sizing guide.
During the lifetime of a patient, it may be necessary to perform a joint replacement procedure on the patient as a result of, for example, disease or trauma. The joint replacement procedure, or joint arthroplasty, may involve the use of a prosthetic implant which is coupled to one of the patient's bones.
During performance of a joint replacement procedure, it is generally important to provide the orthopaedic surgeon with a certain degree of flexibility in the selection of the correct size of prosthetic implant. In particular, the anatomy of the bone to which the implant is to be coupled may vary somewhat from patient to patient. In order to implant a prosthetic joint, it is commonly necessary to prepare the bone to receive the prosthesis. For a prosthetic knee joint, both the distal femur and the proximal tibia may need to be accurately resected to shape the ends of the bones to receive the implants. For preparing the distal femur as a first preparatory step, a transverse surface is formed at the distal end of the femur by performing a first resection. This resection may be located using separate instruments, not described in detail here.
Femoral knee implants are typically provided in a range of standard sizes. Once selected, the femoral implant must be located and oriented to provide appropriate rotational alignment. Correct selection of the size of implant and correct positioning of the femoral implant relative to the natural bone is essential to ensure natural movement of the assembled joint. In particular, the implant must be positioned to provide an appropriate gap between the femur and the tibia when the knee is in extension and in flexion, and to ensure that the surrounding tissues are correctly balanced. It is known to set the rotation of the femoral implant relative to Whiteside's line, which extends from the intercondylar notch to the patella groove. Alternatively, the rotation of the femoral implant may be set relative to the transepicondylar axis, which connects the high points of the epicondyles. Further anatomical reference marks may also be used.
It is known to use a femoral sizing guide mounted on a resected distal femoral surface to measure the size of the distal femur in order to determine the appropriate size of femoral implant. The size of a femoral implant is determined by the anterior-posterior size of the implant measured parallel to an anterior-posterior implant axis. The anterior-posterior implant axis extends perpendicularly from a plane which is normal to a distal resected surface of the femur (when the implant is in its final implanted position) and in contact with the posterior condyles of the femoral implant. The anterior-posterior size of a femoral implant is the distance from this posterior condyle plane to the anterior tip of the implant. For an implant intended to be implanted without any rotation relative to the natural position of the condyles, the anterior posterior size of the femur is measured parallel to an anterior-posterior femoral axis. The anterior-posterior femoral axis extends perpendicularly from a plane which is normal to the distal resected femoral surface and in contact with the natural posterior condyles of the femur. The distal resected femoral surface is usually perpendicular to the anatomical longitudinal axis of the femur. The anterior-posterior size of the femur is the distance from the posterior condyle plane to a planned implanted position of the anterior tip of the implant. The measurement position on the anterior surface of the femur representing the planned implanted position of the anterior tip of the implant varies according to the size of the selected femoral implant. Measurement of the anterior-posterior size of a natural femur will be described below for certain known femoral sizing guides and in greater detail below in connection with embodiments of the present invention.
It is also known to use a femoral sizing guide to specify the required locations of guiding apertures into the femur to secure an appropriate cutting block to the femur for preparing the femur to receive the implant. A surgical instrument set for performing a knee replacement procedure including a femoral sizing guide is marketed by DePuy Orthopaedics, Inc. under the trade mark Sigma High Performance Instruments (Sigma HP Instruments). The Sigma HP instrument set allows surgeons to perform total knee arthroplasty procedures under most surgical approaches.
The Sigma HP femoral sizing guide is used to determine the correct size for the femoral implant and to position guide pins to support a cutting block on the distal end of the femur. A body portion of the femoral sizing guide is seated upon the resected femoral surface. Posterior feet extend from the body underneath the posterior condyles. The femoral sizing guide is correctly located when the posterior condylar surfaces rest upon the feet with Whiteside's line extending generally centrally through the sizing guide. When the feet are correctly located the body can be secured to the bone with pins which extend into the bone through fixed position pin holes. A stylus is coupled to the body such that it can be raised and lowered and locked in position. The stylus tip extends over the anterior cortex of the distal femur. The stylus can also rotate about an axis extending generally parallel to the resected surface and can slide through the coupling to the body along the anterior cortex generally parallel to the longitudinal axis of the femur. The tip of the stylus is positioned upon the anterior cortex of the femur at the intended exit point of the anterior cut for the femoral implant (which corresponds to the implanted position of the anterior tip of the corresponding implant). The height of the stylus tip above the condylar feet corresponds to the anterior-posterior distance and can be read off a scale upon the body. The measured anterior-posterior distance indicates the size of the required implant.
A scale on the stylus indicates the size of implant. The chosen size of the femoral implant determines the size of cutting block to be coupled to the distal transverse surface of the femur. The stylus position sliding through the sizing guide and the height of the stylus above the feet are set to the same value on the respective scales (corresponding to the size of the femoral implant). When the tip of the stylus just contacts the anterior cortex as the stylus sweeps across the anterior cortex, the anterior part of the corresponding size of femoral implant will terminate at the anterior surface of the bone without leaving a notch or an overhang.
There are two alternative surgical approaches for positioning a femoral implant using the Sigma HP instrument set. These are termed “posterior up” and “anterior down”. Posterior up is based upon accurately positioning the cutting block relative to the posterior side of the distal femur (specifically, the posterior condylar surfaces as referenced by the feet of the sizing guide). Anterior down is based upon accurately positioning the cutting guide relative to the anterior side of the distal femur (specifically, the tip of the stylus). For the existing Sigma HP femoral sizing guide, the positioning of guide pins to support the cutting block (for making the anterior, posterior and chamfer cuts) is determined by coupling separate guide blocks to the femoral sizing guide.
The guide blocks and the position at which they couple to the rest of the sizing guide vary according to whether the chosen approach is posterior up or anterior down. Furthermore, the posterior up and anterior down blocks are available in four versions each which relate to different degrees of external rotation of the femoral implant relative to Whiteside's line or the transepicondylar axis (0°, 3°, 5° and 7° of external rotation viewed from the perspective of the surgeon observing the distal end of the femur). For an anterior down approach the guide blocks couple to the sizing guide close to the stylus and comprise wings which extend downwards over the transverse distal surface of the femur and include drill guides for positioning guide pins to support the selected cutting block. The guide blocks further comprises a blade which indicates alignment with Whiteside's line. For a posterior up approach the guide blocks couple to the sizing guide close to the pins securing the body to the bone, above the feet. The posterior up guide blocks comprise wings which extend over the transverse distal surface of the femur and include drill guides. The guide blocks further comprise a reference surface which indicates alignment with the transepicondylar axis.
Once the cutting block pins have been positioned, the femoral sizing guide including the guide block can be removed and the appropriate cutting block (according to the selected size of femoral implant) can be positioned over the guide pins to perform the anterior, posterior and chamfer cuts.
U.S. Pat. No. 6,458,135 (assigned to Howmedica Osteonics Corp.) discloses a femoral sizing guide for determining the required size of femoral implant and for determining the required location of alignment holes for securing a cutting block to the distal end of a femur. The femoral sizing guide comprises a sizing block connected to a foot component for selective pivotal movement about a pivot axis extending axially along the femur. The foot component comprises a pair of feet positioned against the posterior condylar surfaces. The centre of rotation is between the feet. A detent mechanism restricts rotational movement to predetermined angular positions. A locking mechanism is released and the device is manually turned and locked in the required position. The sizing block is positioned upon a resected transverse distal surface of the femur. Rotation of the sizing block relative to the foot component (and hence rotation of the position of the alignment holes to be drilled through the sizing block) allows the rotational position of the femoral implant about the femur to be varied. The size of femoral implant required can be determined either using guide structures upon the sizing block or via a stylus coupled to the sizing block.
For the Howmedica device, the position of the alignment holes is directly referenced to the posterior condylar surfaces through the selection of appropriate drill guide bushings to couple to the sizing block which correspond to the selected size of femoral prosthesis. For the Howmedica instrument set a single cutting block is used for performing an initial anterior cut, and the position of the anterior cut varies as each drill guide bushing adjusts the position of the alignment holes relative to the foot component. Further adjustment of the position of the anterior cut is provided through a sliding coupling between the sizing block and the foot component. The Howmedica femoral sizing block therefore only allows a surgeon to follow a posterior up surgical approach.
U.S. Pat. No. 7,488,324 (assigned to Biomet Manufacturing Corporation) discloses a modular femoral sizing guide which facilitates the selection and orientation of a femoral implant. A base portion is coupled to a resected transverse distal surface of the femur. An extension portion has a pair of feet to be positioned under the posterior condylar surfaces. The extension portion is rotatably coupled to the base portion. The centre of rotation is between the feet. A superstructure portion has a pair of drill guides to prepare alignment holes extending axially into the femur to couple a cutting block to the distal femur. The superstructure further comprises a stylus to determine the required size of the femoral implant.
To position the alignment holes, the Biomet femoral sizing guide comprises a first actuator to rotate the extension portion relative to the base portion (thereby rotating the drill guides about the femoral axis). The sizing guide further comprises a second actuator to adjust the height of the superstructure relative to the base portion (thereby raising or lowering the drill guides relative to the feet).
For the Biomet device, the position of the alignment holes is referenced to the stylus tip by control of the second actuator to lower the stylus and hence the drill guides until the tip of the stylus contacts the anterior cortex. Consequently, the Biomet femoral sizing block therefore only allows a surgeon to follow an anterior down surgical approach.
It is an aim of embodiments of the present invention to obviate or mitigate one or more of the problems associated with the prior art, whether identified herein or elsewhere. In particular it is an aim of embodiment of the present invention to provide a femoral sizing guide in which a rotation mechanism allows portion of the sizing guide to rotate relative to one another while controlling the effect of that rotation upon measurement of the anterior-posterior distance of the femur.